Apparatus and method for image processing, and computer product

ABSTRACT

In an image processing apparatus, a pseudo tone processor performs a pseudo tone process by performing an area coverage modulation expression process with respect to M-bit image data, to thereby convert the M-bit image data into N-bit image data, where N is smaller than M. A lossy compressor performs lossy compression with respect to the N-bit image data, by suppressing fluctuations in a pseudo tone processing frequency within a certain range.

The present application claims priority to and incorporates by referencethe entire contents of Japanese priority documents, 2004-079291, filedin Japan on Mar. 18, 2004, and 2004-322716 filed in Japan on Nov. 5,2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method for imageprocessing, and a computer product in which a tone corresponding to bitsin an image in preserved, and mean value preservability and the visualperformance can be substantially obtained.

2. Description of the Related Art

Conventionally, in a color copying machine having an image storage unitsuch as a hard disk, an image read by a scanner is stored in the harddisk, and transmitted to external equipment. Considering storagecapacity and transmission speed, images are generally compressed beforestoring or transmitting. Another conventional apparatus is one in whichlossy compression is employed, as represented by Joint PhotographicExperts Group (JPEG). In the lossy compression method, high compressionefficiency can be expected with a little effect on the image quality,depending on the image. The JPEG images can be displayed on a displayunit using general-purpose software on a personal computer (PC), andhence its accessibility is excellent. To output an image as a copiedimage, an image stored must be read out, and subjected to imageprocessing for reproducing the image.

The method in JPEG adapted by most types of general-purpose software isa baseline method of JPEG, and its limitation is that the number of bitsin the baseline method is 8 bits for each color. However, sometimes, itis desired to store an image expressed by more than 8 bits. The mostnoticeable example is when the number of input bits in an input unit(scanner) is larger than 8 bits. In fact, a 12-bit input unit (scanner)is available in the market. Using 12 bits in the input stage reducesquantization errors. Further, when processing such as nonlinearoperation and color space conversion is performed during scanner inputand storing of images, a number of bits may be required to approximatethese conversions to reversible conversion. This is because quantizationerrors should be reduced to approximate the conversion to the reversibleconversion. In the nonlinear operation, in which complete reversibleconversion requires real number calculation, as the number of bitsincreases, quantization errors decreases, and approximates to the realnumber calculation.

The color space conversion method is specifically explained here. It isan effective technique to store the color space signals in a hard disk,and does not depend on devices such as an input unit and an output unit.Moreover, an advantage of the color space conversion is that colorreproducibility does not change even when the input unit or the outputunit changes. sRGB (standard-RGB), sYCC, and scRGB are well-knowndevice-independent color spaces. However, sRGB has a narrow color gamut,requires gamut compression with respect to the input image from ascanner, and has poor color reproducibility. On the other hand, sYCC andscRGB have substantially sufficient color gamut, but due to problemssuch as the size of the color gamut and quantization errors, it isnecessary to express an image with a larger number of bits than in sRGB.To store a JPEG-compressed image expressed by a large number of bits, abit number reduction unit is necessary. However, if low-order 2-bits aresimply omitted, there is a problem in that a pseudo profile that is notvisible in the case of 10-bit expression is visible in some images.

-   -   (1) Japanese Patent Application Laid-open No. H2-153676        discloses an image forming apparatus that converts high-tone        image data of N bits into N-m bit data. In this technique,        low-order M bits of the N bit data are binarized, and high-order        N-m bits of the N bit data are combined with the binarized        one-bit data, to generate N-m bit data. Expressing the low-order        M bits corresponding to the omitted portion by pseudo tone and        binarizing the M bits, enables to store the tone corresponding        to N bits by area coverage modulation, and to thus prevent        occurrence of a pseudo profile. Such a technique has been in use        when there is a limitation in the number of tones that can be        displayed by a display unit, or when there is a limitation in        the number of bits to be transmitted, and the like, and        particularly for displaying on a television screen.

However, reduction in number of bits executed on the television is notexecuted by assuming subsequent lossy compression because a lossycompressor is not included. Even in the conventional art, in which thenumber of bits is reduced by using pseudo tone processing for datatransmission, lossless compression is assumed in most cases. On theother hand, an apparatus having a lossy compressor such as the JPEG,executes lossy compression after reducing the number of bits from 10bits to 8 bits. If pseudo tone processing is performed with respect toan image to reduce the number of bits from 10 bits to 8 bits and thenthe JPEG processing is performed, there is a problem in that informationfor the low-order 2 bits subjected to the pseudo tone processing isalmost lost, depending on the pseudo tone processing. In the lossycompression, because the compressibility is increased by unsmoothing aquantization step for high frequency components, the loss occurs whenthe information for the low-order 2 bits is present just in the placewhere it is unsmooth in the quantization step.

-   -   (2) Further, there are few apparatuses that perform lossy        compression such as JPEG after performing the pseudo tone        processing. Japanese Patent Application Laid-open No. H8-317393        discloses an image forming technique in which JPEG compression        is performed after error diffusion (or dither processing). In        this technique, after the number of bits is reduced from 8 bits        to 4 bits by error diffusion, “0000” is added to the low-order        bits to form 8-bit data, and the data is JPEG-compressed and        stored in an image memory. However, the object of this technique        is to reduce the storage capacity of the image memory (a memory        used for a blocking unit 61 in FIG. 3), and not preservation of        tone after the lossy compression, nor preservation of tone        corresponding to 10 bits, which is the number of bits equal to        or larger than the bits at the time of JPEG compression. There        is also a problem that when compressed data is transmitted to        external equipment and displayed on a monitor a halftone image        is visible (noticeability of error diffusion or dither basic        tone).    -   (3) Japanese Patent Application Laid-open No. 2001-277602        discloses an apparatus in a printer system that performs image        processing and compression processing suitable for each object,        where the halftone processing is included in the image        processing, and performs error diffusion and dither processing.        In some cases, the compressibility is switched.    -   (4) Further, Japanese Patent Application Laid-open No. H9-149260        discloses an image forming apparatus that performs compression        after the dither processing, and in which an amount of generated        code of compressed data is monitored. As long as the amount of        code is within an allowable range, multi-level dither processing        and lossless compression are performed. Only when the amount of        code exceeds the allowable range, the processing method is        switched to binary dither processing and lossy compression.

The technique in (1) is a practical technique for displaying images onthe television screen, but the number of bits is not reduced by assumingthe lossy compression, because the lossy compressor is not provided inthe stage after reduction of number of bits. The technique in (2) is forreducing the storage capacity of the image memory, and does not aim atpreserving the tone after the lossy compression, nor at preserving thetone corresponding to 10 bits, which are larger than those at the timeof JPEG compression. Therefore, the problem that a halftone imageappears when compressed data is transmitted to external equipment anddisplayed on a monitor, still persists. The techniques in (3) and (4)include lossy compression after the halftone processing only when thecompressibility is prioritized, and hence, do not include a function ofmaintaining the tone.

SUMMARY OF THE INVENTION

An apparatus and method for image processing and computer product aredescribed. In one embodiment, the image processing apparatus comprises apseudo tone processor that performs a pseudo tone process by performingan area coverage modulation expression process with respect to M-bitimage data to convert the M-bit image data to N-bit image data, where Nis smaller than M, and a lossy compressor that performs lossycompression with respect to the N-bit image data by suppressingfluctuations in a pseudo tone processing frequency within a certainrange.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a functional block diagram of an image forming apparatusaccording to an embodiment of the present invention;

FIG. 1B is a flowchart of process procedures, from image input by ascanner to filter processing, performed in the image processingapparatus;

FIG. 1C is a flowchart of process procedures from JPEG compression toimage output by a printer;

FIG. 2A is a functional block diagram of a pseudo tone processor;

FIG. 2B is a diagram to illustrate a procedure of the pseudo toneprocessing;

FIG. 2C illustrates an example of a threshold value used by a thresholdprocessor;

FIG. 3 is a functional block diagram of a JPEG compression unit;

FIG. 4 is a diagram to illustrate a process procedure of cutting outimage data into block units;

FIG. 5 illustrates examples of standard quantization tables;

FIGS. 6A and 6B are diagrams to illustrate degradation in an imagebefore and after JPEG compression and expansion;

FIG. 7 illustrates an example of a 1-bit pattern after threshold valueprocessing by the threshold processor;

FIG. 8 illustrates a second example of a 1-bit pattern after thethreshold value processing;

FIG. 9 illustrates a third example of a 1-bit pattern after thethreshold value processing;

FIG. 10 is a diagram to illustrate a relation between visual performanceand average density preservability by JPEG;

FIG. 11 is a functional block diagram of an image processing apparatusaccording to a first embodiment;

FIGS. 12A and 12B illustrate a difference in average densitypreservability according to a compression level of the image processingapparatus shown in FIG. 11;

FIG. 13A illustrates changes in a frequency characteristic of a ditherpattern corresponding to the compressibility in the image processingapparatus shown in FIG. 11;

FIG. 13B is a diagram to illustrate the frequency characteristic of thedither pattern;

FIG. 14 is a graph to illustrate that in addition to switchover of thefrequency of the dither pattern, amplitude is changed in the imageprocessing apparatus shown in FIG. 11;

FIG. 15 is a diagram to illustrate a reflection method of omitted bitsin the image processing apparatus shown in FIG. 11;

FIG. 16 is a functional block diagram of an image processing apparatusaccording to a second embodiment;

FIG. 17A is another diagram to illustrate the relation between visualperformance and average density preservability by JPEG;

FIG. 17B is a functional block diagram of an automatic image areadetector;

FIG. 17C is a diagram to illustrate an operation of a combining unit inthe automatic image area detector;

FIG. 18 is a functional block diagram of an image processing apparatusaccording to a third embodiment; and

FIG. 19 illustrates an example of a hardware configuration of the imageprocessing apparatus.

DETAILED DESCRIPTION

An embodiment of the present invention to at least solve the problemsdescribed above in the conventional technology.

An image processing apparatus according to an embodiment of the presentinvention includes a pseudo tone processor that performs a pseudo toneprocess by performing an area coverage modulation expression processwith respect to M-bit image data, to thereby convert the M-bit imagedata to N-bit image data, wherein N is smaller than M; and a lossycompressor that performs lossy compression with respect to the N-bitimage data by suppressing fluctuations in a pseudo tone processingfrequency within a certain range.

An image processing apparatus according to another embodiment of thepresent invention includes a pseudo tone processor that performs apseudo tone process by performing an area coverage modulation expressionprocess with respect to image data, to thereby reduce a number of bitsof the image data; a lossy compressor that performs lossy compressionwith respect to the image data subjected to the pseudo tone process; anedge extractor that extracts an edge amount from the image data beforethe lossy compression; and an image synthesizing unit that synthesizesthe image data subjected to the lossy compression and the edge amountextracted.

An image processing method according to still another embodiment of thepresent invention includes pseudo tone processing by performing an areacoverage modulation expression process with respect to M-bit image data,thereby converting the M-bit image data to N-bit image data, wherein Nis smaller than M; and performing lossy compression with respect to theN-bit image data by suppressing fluctuations in a pseudo tone processingfrequency within a certain range.

An image processing method according to still another embodiment of thepresent invention includes pseudo tone processing by performing an areacoverage modulation expression process with respect to image data,thereby reducing a number of bits of the image data; performing lossycompression with respect to the image data subjected to the pseudo toneprocess; extracting an edge amount from the image data before theperforming; and synthesizing the image data subjected to the lossycompression and the edge amount extracted.

Computer-readable recording media according to other embodiments of thepresent invention store thereon a computer program that implement on acomputer the above methods according to one embodiment of the presentinvention.

The other embodiments, features, and advantages of the present inventionare specifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

Exemplary embodiments of the present invention will be explained belowwith reference to the accompanying drawings.

FIG. 1A is a functional block diagram of an image forming apparatusaccording to an embodiment of the present invention.

An image processing apparatus 15 according to the embodiment is providedin an image forming apparatus 100. The image processing apparatus 15includes a pseudo tone processor a 5, a JPEG compression unit 6, a JPEGexpansion unit 7, and a hard disc drive (HDD) 13 that at leasttemporarily stores image data compressed by the JPEG compression unit 6.The HDD 13 transfers data via an external interface (I/F) 14 that is aninterface to external equipment.

FIG. 1B is a flowchart of process procedures, from image input by ascanner to filter processing, performed in the image processingapparatus. In the image forming apparatus 100, a scanner 1 obtains acolor image signal formed of red, green, and blue (RGB) (step S101), ascanner gamma correction unit 2 performs a gamma table conversion (stepS102), and a color correction unit a 3 corrects the characteristics ofthe scanner to output a device-independent YCbCr signal (sYCC signal)(step S103). The scanner gamma correction unit 2 performs Y linearconnection to improve the precision of the color correction unit a 3.

The color correction unit a 3 performs hue split-type color correction.A 10-bit YCbCr signal is output with respect to 8-bit signal of RGB. ThesYCC (standard-YCbCr) is a standard color space, in which a color gamutis widened with respect to sRGB obtained by linear transformation of thesRGB that is a standard color space for color display. However, the sYCCrequires a larger number of bits than the sRGB, to set the tone thereofto the same level as that of sRGB. Therefore, when the sRGB requires 8bits respectively, the sYCC requires about 10 bits respectively.

A filtering processor 4 performs filtering including smoothing and edgereinforcement with respect to the sYCC 10-bit signals, and the pseudotone processor a 5 reduces the number of bits, by area coveragemodulation, to output 8-bit sYCC signals (step S104).

FIG. 1C is a flowchart of process procedures until a printer outputs theimage after a JPEG compression unit has received an sYCC signal, withthe number of bits thereof reduced by a pseudo tone processor. The JPEGcompression unit 6 performs JPEG compression with respect to the sYCC8-bit signal in which the number of bits has been reduced (step S201),and stores the compressed data in the HDD 13 (step S202). In the case ofa reproduction procedure in the normal image formation, the compresseddata stored once is read immediately, and the JPEG expansion unit 7performs expansion processing (step S203), a color correction unit b 8performs color correction (step S204), and a UCR/black generating unit 9performs UCR/black generation processing (step S205) to convert the datato a CMYK signal corresponding to the toner color of the printer. Aprinter gamma correction unit 10 performs gamma correction (step S206),a pseudo tone processor b 11 performs pseudo tone processing (stepS207), and a printer 12 outputs an image (step S208).

The pseudo tone processor b 11 performs dither processing or errordiffusion processing for printer output with respect to the image data,to create the number of bits that are required for printer output (4bits). The compressed data stored in the HDD 13 may also be transmittedfrom the external I/F 14 to the external equipment, or input from theexternal equipment, other than the normal copying procedure. Forexample, an image transmitted to an external PC may be displayed on acolor monitor for observation and editing.

FIG. 2A is a functional block diagram of the pseudo tone processor. Thepseudo tone processor a 5 converts 10 bits to 8 bits. A low-ordertwo-bit selector 52 selects the low-order two bits to be omitted, and athreshold processor 53 binarizes the low-order two bits selected, into a1-bit signal. The threshold processor 53 uses a threshold value,corresponding to a pixel position, which is set in a threshold matrix55. A high-order 8-bit selector 51 selects the high-order 8 bits of the10 bits. An adder 54 adds the 1-bit signal to the least significant bit(LSB) of the high-order 8 bits selected.

FIG. 2B is a diagram to illustrate a procedure in which the pseudo toneprocessor performs pseudo tone processing to reduce the number of bits.FIG. 2C illustrates an example of a threshold value used by thethreshold processor. The procedure in the pseudo tone processingaccording to the embodiment will be specifically explained withreference to FIGS. 2A to 2C.

As an example, when a 10-bit binary input is “1000000011” (515 indecimal), the high-order 8-bit selector 51 selects “10000000”, being thehigh-order 8 bits (step S301). The low-order 2-bit selector 52 selects“11”, being the low-order 2 bits (step S302). The threshold processor 53compares the low-order 2 bits “11” (3 in decimal) with a thresholdvalue. If larger than the threshold value, the threshold processor 53outputs “1”, and if smaller than the threshold value, the thresholdprocessor 53 outputs “0” (step S303). If the threshold value is 2 indecimal, the output from the threshold processor 53 becomes “1”. Anadder 54 adds high-order 8 bits, “10000000” , and “1” from the thresholdprocessor 53, to output “10000001” (step S304).

The threshold value is different depending on the pixel position. Forexample, when the value “1000000011” is input to the threshold processor53 for all of 2×2 pixels, if the threshold value is changed for eachpixel as in an example shown in FIG. 2C, the output from the pseudo toneprocessor a 5 becomes “10000001” (129 in decimal) for the upper leftpixel, “10000001” (129 in decimal) for the upper right pixel, “10000001”(129 in decimal) for the lower left pixel, and “10000000” (128 indecimal) for the lower right pixel. The mean value of 2×2 pixels is128+(¾) in decimal, and (128+(¾))×4=515 is calculated as a mean value,thereby storing the mean value of 515. Here, (128+(¾)) is multiplied by4 for considering the value as a 10-bit value.

FIG. 3 is a functional block diagram of the JPEG compression unit 6.FIG. 4 is a diagram to illustrate the process procedure of cutting outimage data into block units. The JPEG method is an internationalstandard method, and the image is cut out in a unit of 8×8 pixel asshown in the figure. The JPEG compression unit 6 includes a blockingunit 61, a discrete cosine transform (DCT) unit 62, a quantization unit63, and a Huffman encoder 64. The blocking unit 61 cuts out the image ina unit of 8×8 pixels, and the DCT unit 62 performs discrete cosinetransform with respect to the cut out DCT block data, and changes thedata to a frequency space.

The quantization unit 63 that performs quantization processing, whichcontrols a compression level, will be explained later. A standardquantization table 66 includes separate standard quantization tables forY components and CbCr components.

FIG. 5 illustrates examples of standard quantization tables Qij. Theupper left corner Q00 of the 8×8 block is a parameter used forquantization of direct current components (DC components), and theremainder is a parameter used for quantization of alternating currentcomponents (AC components). The closer to the upper left corner, thelower the frequency component, and the closer to the lower right corner,the higher the frequency component. Quantization is performed bydividing a DCT coefficient of 8×8, being an output value from the DCTunit 62, by a quantization table value Q′ij. The larger the quantizationtable value Q′ij, the larger the loss due to quantization (=largequantization), and it is the old trick to increase the compressibilityby increasing quantization of the high frequency components as shown inFIG. 5, with little influence to the image quality.

With respect to the standard quantization table Qij, the quantizationtable value Q′ij actually used in quantization is determined by thefollowing expression:Q′ij=Qij×sf,where sf denotes a fixed parameter set in a scaling factor setting unit67, and is set within a range of 0<sf<1. The smaller the sf, the lowerthe compressibility and the higher the image quality.

FIGS. 6A and 6B are diagrams to explain degradation in an image beforeand after JPEG compression and expansion. Not only in the JPEG but alsowith lossy compression, the high frequency components are attenuated,and hence, a dull image with a smooth corner is obtained, as observed atthe micro level. FIG. 6A illustrates degradation in an image withdensity fluctuations in low frequencies, and FIG. 6B illustratesdegradation in an image with density fluctuations in high frequencies.The density axis is in the vertical direction in the figure. As shown inFIGS. 6A and 6B, the image becomes dull in the image after compressionand expansion. While a difference in tone (a fluctuation margin ofdensity fluctuations) is stored in the case of FIG. 6A, it is not in thecase of FIG. 6B. If there is a larger attenuation in the high frequencycomponents due to compression than the attenuation shown in the figure,the amplitude in the density fluctuations shown in FIG. 6B becomes zero,and undulations disappear. That is, when the difference in tone of thedensity fluctuations is 1, the difference in tone becomes 0 after thecompression. For example, when the density on the low density side inthe density fluctuations before the compression is 10, and the densityon the high density side is 11, then after the compression andexpansion, density fluctuation becomes zero, and the density of allpixels becomes 10 or 11, and the mean value (in this case, 10.5) isstored in the case of FIG. 6A, but may not be stored in the case of FIG.6B, according to the degree of compression.

FIGS. 7 to 9 illustrate examples of the 1-bit patterns after thresholdvalue processing by the threshold processor 53. For the brevity ofexplanation, an instance in which all 8×8 pixels have the same 10-bitvalue is considered here. For example, when the low-order 2 bits are“01”, the 1-bit pattern in 1-1 of FIG. 7 is added to the LSB of thehigh-order 8 bits, when the low-order 2 bits are “10”, the 1-bit patternin 1-2 of FIG. 7 is added to the LSB of the high-order 8 bits, and whenthe low-order 2 bits are “11”, the 1-bit pattern in 1-3 of FIG. 7 isadded to the LSB of the high-order 8 bits, thereby storing informationcorresponding to 10 bits as an average density of the 8×8 pixels. Athreshold matrix included in the threshold matrix 55 (FIG. 2) is set toa threshold matrix corresponding to the 8×8 pixels generated by such adither pattern. The same applies to FIGS. 8 and 9, but the ditherpattern in FIG. 8 can be realized by a threshold matrix corresponding to4×4 pixels.

In deciding the dither pattern, if the human visual performance is takeninto consideration, the low frequency component is noticeable and isperceived as noise, whereas the high frequency component is notnoticeable. Taking the JPEG compression into consideration, there is aproblem in that as the frequency becomes higher, it becomes moredifficult to preserve the average density due to the reason explainedabove.

FIG. 10 is a diagram to explain the relation between the visualperformance and the average density preservability by the JPEG. Afrequency A corresponds to an allowable threshold value in view of thevisual performance, and a frequency B corresponds to an allowablethreshold value in view of the average density preservability. If thedither pattern is set to have the frequency characteristic in a range ofhigher than A and lower than B, both allowable threshold values can besatisfied. Because the average density preservability depends on thequantization table value Q′ij for compression, and the visualperformance depends on an observation distance or the like, the relationbetween these cannot be mentioned indiscriminately. In relation to theaverage density preservability, for example, in the case of the ditherpattern shown in FIG. 7, many pieces of information are present at afrequency corresponding to a quantization table value Q′11 for luminancecomponents, and the value of Q′11 largely contributes to the averagedensity preservability. In the case of the dither patterns shown inFIGS. 8 and 9, many pieces of information are present at a frequencycorresponding to Q′33, and the value of Q′33 largely contributes to theaverage density preservability.

In order to preserve the average density corresponding to 10 bits withinan area corresponding to the JPEG 8×8 blocking, it is desirable topreserve the average density at intervals of 0.25. For example, betweenthe average densities of 10 and 11, it is desirable to preserve threeaverage density steps of 10.25, 10.50, and 10.75.

On the other hand, the threshold value in view of the average densitypreservability (which can be regarded also as an allowable thresholdvalue with respect to Q′ij) can preserve the tone without beingidentical to the adjacent tone, so long as it is a threshold value thatcan preserve at least 50% of the average density difference 0.25, fromthe adjacent tone (a density difference from the adjacent tone remainsat 0.125 or higher, even after compression and expansion).

In the embodiment, the average density preservability is used forrepresentation. However, the pseudo tone processor a 5 and the JPEGcompression unit 6 use an average color difference instead of theaverage density, to execute the processes with respect to a colordifference signal CbCr. In any case, in one embodiment, the “mean value”is preserved, and there is no supplementation when a signal attribute isdifferent. The same applies in examples described below.

According to the embodiment, the pseudo tone processor a 5 performs thepseudo tone processing to reduce the number of bits, and then in theprocedure for JPEG compression, a mean value within the areacorresponding to the JPEG block size is preserved even after compressionand expansion, and the frequency for pseudo tone processing is setwithin a favorable range also with respect to the visual performance.Therefore, the tone corresponding to the number of bits larger than 8bits can be roughly preserved, thereby suppressing occurrence of noiseand texture perceivable by the eyes due to the pseudo tone processing,even when an image stored in the HDD is displayed on the monitor.

FIG. 11 is a functional block diagram of the image processing apparatus15 according to a first embodiment. The image processing apparatus inthis embodiment has a function for specifying the compression level viaan operation panel 21. More specifically, the scaling factor in a JPEGcompression unit 23 can be changed, and hence, the compression level canbe changed. In comparison with FIG. 3, the configuration is such thatthe scaling factor setting unit 67 becomes variable.

FIGS. 12A and 12B illustrate a difference in the average densitypreservability according to the compression level of the imageprocessing apparatus. When the compression level of the image processingapparatus changes, the average density preservability also changes. Inthe case of a low compressibility (small quantization), the averagedensity preservability from the intermediate frequency to the highfrequency improves, and in the case of a high compressibility (largequantization), the average density preservability decreases.

FIG. 13A illustrates changes in the frequency characteristic of a ditherpattern corresponding to the compressibility in the image processingapparatus. Thus, by changing the frequency characteristic of the ditherpattern corresponding to the compressibility, the average densitypreservability can be kept constant regardless of the compressibility.

FIG. 13B is a diagram to illustrate the frequency characteristic of thedither pattern. The frequency characteristic of the dither patterncharacterizes whether the frequency of an image generated by the ditherpattern is high or low. For example, the operation by a mask 130 a and amask 130 b of two dither threshold values shown in FIG. 13B is studied,as in the above explanation. If 129 pixels are expressed by black, and128 pixels are expressed by white, the results of applying the masks 130a and 130 b of the dither threshold value become an image 131 a and animage 131 b, respectively. The images 131 a and 131 b are the results ofhalftone processing, and are the same as seen from the mean value of 4×4pixels, but the image 131 a has a high frequency, and the image 131 bhas a low frequency. That is, the threshold masks 130 a and 130 b of thedither pattern shown in FIG. 13B respectively form a high frequencyimage 131 a, and a low frequency image 131 b.

In the case of high compressibility, as shown in FIG. 12B, the relationbetween A and B is reversed, and setting of the frequency characteristicin the range of higher than A and lower than B may not be possible. Inthis case, either of the visual performance and the average densitypreservability is given priority, to set the frequency of the ditherpattern to either A or B. However, when A and B are reversed, a pseudotone processor a 22 is substantially turned off, to simply omit thelow-order 2 bits. By giving priority to the visual performance, theaverage density preservability is sacrificed. However, the averagedensity preservability can be prevented from becoming unsatisfactorilyinsufficient, and unnecessary texture can be prevented from becomingnoticeable after the JPEG compression and expansion.

FIG. 14 is a graph to illustrate that in addition to switchover of thefrequency of the dither pattern, the amplitude is changed in the imageprocessing apparatus. In FIG. 14, it is effective to change theamplitude, in addition to switchover of the frequency of the ditherpattern. That is, if the amplitude is increased, even if the tonedifference decreases as shown in FIG. 6B, the tone difference hardlybecomes zero, and hence, it is advantageous for preserving the averagedensity.

FIG. 15 is a diagram to illustrate a reflection method of omitted bitsin the image processing apparatus. In the embodiments explained above,the omitted bits are reflected by adding the bits to the LSB of thehigh-order 8 bits (A in FIG. 15), to increase the amplitude. However,the omitted bits may be reflected on bits other than the LSB (B in FIG.15). Furthermore, the tone corresponding to 10 bits can be preserved bythe area coverage modulation.

For example, adding the dither pattern 3-2 in FIG. 9 to the LSB, andadding the dither pattern 3-1 in FIG. 9 to a position one bit higherthan the LSB have the same meaning as the average density of 8×8 pixels.

According to this embodiment, even an apparatus having a functioncapable of setting the compressibility can roughly preserve the tonecorresponding to the number of bits larger than 8 bits, regardless ofthe compressibility, and when an image stored in the HDD is displayed onthe monitor, the noise and texture perceivable by the eyes due to thepseudo tone process, is not generated.

FIG. 16 is a functional block diagram of an image processing apparatusaccording to a second embodiment. In the image processing apparatus 15in this embodiment, an operation panel 31 accepts the scaling factor tobe set, and a pixel density to be set of an image stored in the HDD, animage transmitted from external equipment, and an image output from aprinter. A document size detector 36 detects the document sizeautomatically. An automatic image area detector 38 detects a patternarea in an image to detect the size of the pattern area. The frequencysetting of a pseudo tone processor a 32 is switched over as in thesecond embodiment, corresponding to the value set or the value detectedby these units.

FIG. 17A is another diagram to illustrate the relation between thevisual performance and the average density preservability by JPEG.Generally, the larger the document size is, the larger the observationdistance becomes. For example, assuming that FIG. 10 represents therelation between the visual performance and the average densitypreservability with respect to the postcard size, the relation withrespect to the A3 size is as shown in FIG. 17A, in which the flexibilityimproves for frequency setting, and hence the movement is advantageous.

On the other hand, even if the document size is large, and if the actualpattern area is small, the observation distance does not increase.Further, the demand for suppressing image quality degradation bypreserving the average density is basically because of a pattern arearepresented by a gradation image. When character images occupy a largearea, it is generally not necessary to switchover the frequency settingaccording to the document size.

The automatic image area detector 38 shown in FIG. 16 automaticallyextracts a pattern as an image area, in a rectangular area surrounding ahalftone dot pattern and a photograph pattern by integrating patternareas detected in a unit of pixel or in a unit of block. The automaticimage area detector 38 detects pattern areas of an image in arectangular shape, and provides control such that when the number ofpixels detected as a pattern area is large, the frequency settingsimilar to the case of large document size is performed, but when thenumber of pixels is small, even if the document size is large, frequencysetting is not performed in such a manner.

FIG. 17B is a functional block diagram of the automatic image areadetector. The automatic image area detector 38 includes a halftone dotblock determination unit 380, a photographic block determination unit384, an OR unit 390, a combining unit 392, and a rectangular areadetermination unit 394.

A peak pixel detector 381 in the halftone dot block determination unit380 determines, from the density relation with surrounding pixels,whether a target pixel is a pole indicating a peak in density changes.With reference to M×M pixels (in this case, 5×5 pixels) the peak pixeldetector 381 detects a central pixel as a peak pixel, when an absolutevalue of the density difference between a mean value of two pixel levelslocated in symmetric positions putting the central pixel therebetweenand the central pixel is larger than a predetermined threshold value.

A counter unit 382 counts the number of peak pixels in a block, in aunit of block of a predetermined size. When the number of peak pixels islarger than a predetermined threshold value, a determination unit 383determines that the target block is a halftone dot block. For example,assuming that one block has 8×8 pixels, when there are 12 or more peakpixels in a block, the block is determined to be a halftone dot block.

In the photographic block determination unit 384, a ternarizing unit 385performs a ternarizing process with two threshold values. A patternmatching unit 386 performs pattern matching with respect to anintermediate level pixel of the ternarized values. For example, when all3×3 pixels are intermediate level pixels, the target pixel is output asa photographic pixel. A counter unit 387 counts the number ofphotographic pixels in a block, in a unit of block of a predeterminedsize. When the number of photographic pixels is larger than apredetermined threshold value, a determination unit 388 determines thatthe target block is a photographic block. For example, assuming that oneblock has 8×8 pixels, when there are 30 or more photographic pixels in ablock, the block is determined to be a photographic block.

The OR unit 390 designates the target block determined as a halftone dotblock by the halftone dot block determination unit 380, and the targetblock determined as a photographic block by the photographic blockdetermination unit 384, as an active block.

FIG. 17C is a diagram to illustrate an operation of a combining unit inthe automatic image area detector for determining the active block. Thecombining unit 392 refers to upper, left, right, and lower blocks a, b,c, and d with respect to the target block e shown in FIG. 17C. When aand b are active blocks, a and c are active blocks, d and b are activeblocks, or d and c are active blocks, the combining unit 392 determinesthat the target block is an active block. This process is repeated apredetermined number of times (in this case, 10 times). When the size ofthe active block is equal to or larger than a predetermined size (forexample, 20 mm×20 mm) after processing by the combining unit 392, therectangular area determination unit 394 determines that the block is apattern area.

The same applies to the pixel density, and if the pixel density is high,the relation between the visual performance and the average densitypreservability becomes as shown in FIG. 17A.

Thus, when a scaling unit 37 that performs scaling is provided after theexpansion process by a JPEG expansion unit 34, the frequency of thedither pattern added by the pseudo tone processor a 32 that observes theimage as a printer output image is different from the frequencyimmediately after the processing by the pseudo tone processor a 32.Taking this matter into consideration, it is an effective method tochange over the frequency of the dither pattern beforehand, based on thescaling factor.

The document size detector 36 and the automatic image area detector 38may specify the document size and the image area via the operation panel31 as for the scaling factor and the like. Alternatively, when theautomatic image area detector 38 detects the image area automatically, amethod of detecting the image area from a prescanned image or from animage stored in the memory is effective.

According to this example, even when the apparatus has a function forsetting or automatically obtaining parameters that affect the visualperformance such as the document size, the pattern area size, thescaling factor, and the pixel density, the tone corresponding to thenumber of bits larger than 8 bits can be roughly preserved as in thefirst embodiment, regardless of the parameter values. Displaying theimage stored in the HDD on the monitor further prevents the occurrenceof noise and texture perceivable by the eyes due to the pseudo toneprocess.

FIG. 18 is a functional block diagram of an image processing apparatusaccording to a third embodiment. The image processing apparatus 15 inthis embodiment includes an edge extractor 44 that generates a signalcapable of responding by an edge of an image, and an image synthesisunit 47 that synthesizes an image after the JPEG expansion. As shown inFIGS. 6A and 6B, in an image after compression and expansion, highfrequency components attenuate and edge components are weakened.Therefore, the weakened edge components (ideally, a difference betweenbefore and after the compression, but such a high precision is notrequired) are generated from the signal before the JPEG compression,stored, and subjected to image synthesis, to restore the sharpness ofthe edge. In FIG. 18, the edge extractor 44 uses a signal before apseudo tone processor a 41, but a signal immediately before the JPEGcompression may be used therefor.

For example, consider that the number of bits before the pseudo toneprocess is 30 bits (10 bits×3 colors), the number of bits after thepseudo tone process is 24 bits (8 bits×3 colors), and the number of bitsof the edge signal, being the edge extraction result, is 2 bits.Comparing this with an instance in which the image data for 30 bits isstored in an HDD 18 without performing the pseudo tone process, even ifthe number of bits after the pseudo tone process and the number of bitsof the edge signal are added up, the effect of reducing the number ofbits as an image stored in the HDD can be comprehensively maintained.

Thus, according to the third embodiment, in addition to the effect ofpreserving the tone as in the second embodiment, there is an effect ofpreserving the sharpness of the edge, being a problem in the lossycompression.

FIG. 19 illustrates an example of a hardware configuration of the imageprocessing apparatus according to the embodiment. The image processingapparatus can be realized by executing a program, prepared in advance,in a computer system such as a personal computer or a workstation. In acomputer 200, the whole apparatus is controlled by a central processingunit (CPU) 101. A read only memory (ROM) 102, a random access memory(RAM) 103, a hard disk drive (HDD) 104, a graphic processor 105, and aninput interface 106 are connected to the CPU 101 via a bus 107. Theprogram for the operating system (OS) and at least a part of applicationprograms to be executed by the CPU 101 are stored in the ROM 102 and theRAM 103. The RAM 103 also stores various types of data required for theprocessing by the CPU 101. The OS, various kinds of driver programs,application programs, and detected data are stored in the HDD 104.

The graphic processor 105, the input interface 106, and an image formingunit 114 are connected to the bus 107. A monitor 111 is connected to thegraphic processor 105. The graphic processor 105 allows an image to bedisplayed on the monitor 111 according to an instruction from the CPU101. A keyboard 112 and a mouse 113 are connected to the input interface106. The input interface 106 transmits a signal transmitted through thekeyboard 112 and the mouse 113 to the CPU 101 via the bus 107. The imageforming unit 114 forms an image.

The processing functions in the embodiments described above can berealized by such a hardware configuration. In order to realize theembodiments on the computer 200, the driver program is installedtherein.

The image processing program executed by the image processing apparatusin the embodiments is recorded on a computer-readable recording mediumsuch as a CD-ROM, a floppy (registered trademark) disk, or a digitalversatile disk (DVD), in a file in an installable format or anexecutable format.

The image processing program in the embodiments may be provided anddistributed by storing the program on a computer connected to a networksuch as the Internet and having the program downloaded via the network.

Thus, according to one embodiment of the present invention, in an imageformation process in which lossy compression is performed after thepseudo tone processing, if the number of bits at the time of lossycompression is N, then the tone corresponding to M bits, where M islarger than N, can be roughly preserved even after compression.

Moreover, if the lossy compression is block compression in whichcompression is performed in a unit of block, a mean value correspondingto the M bits is preserved.

Furthermore, even after the lossy compression, a difference between themean value of adjacent tones in the M-bit image data can be preservedmore than 50%.

Moreover, in an apparatus having a compression level setting unit, acertain mean value can be preserved regardless of the compression level,and the tone corresponding to M bits can be preserved at all times.

Furthermore, in an apparatus having a function for variably setting aparameter of the image size, which affects the visual performance, boththe mean value preservability and the visual performance can besubstantially obtained regardless of the parameter, and the tonecorresponding to M bits can be preserved.

Moreover, in an apparatus having a function for variably setting aparameter of the pattern area size, which affects the visualperformance, both the mean value preservability and the visualperformance can be substantially obtained regardless of the parameter,and the tone corresponding to M bits can be preserved.

Furthermore, in an apparatus having a function for variably setting aparameter of the scaling factor for scaling an image, which affects thevisual performance, both the mean value preservability and the visualperformance can be substantially obtained regardless of the parameter,and the tone corresponding to M bits can be preserved.

Moreover, in an apparatus having a function for variably setting aparameter of the pixel density, which affects the visual performance,both the mean value preservability and the visual performance can besubstantially obtained regardless of the parameter, and the tonecorresponding to M bits can be preserved.

Furthermore, the compression level, the image size, the pattern areasize, the scaling factor, and the pixel density are parameters forvarying the pseudo tone processing frequency, and the pseudo toneprocessing is performed only when at least one parameter from among thecompression level, the image size, the pattern area size, the scalingfactor, and the pixel density, is within a predetermined range.

Moreover, the flexibility in setting the frequency that can obtain boththe mean value preservability and the visual performance improves byvarying the amplitude as well as the frequency for the pseudo toneprocessing.

According to another embodiment of the present invention, the sharpnessof the edge after the lossy compression and expansion can be restored inaddition to preserving the mean value, degradation in an image due tothe lossy compression can be further reduced, and an excellent image canbe obtained.

Moreover, number of bits in the image reduces, and the HDD storagecapacity can also be reduced.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

1. An image processing apparatus comprising: a pseudo tone processor toperform a pseudo tone process by performing an area coverage modulationexpression process with respect to M-bit image data, to thereby convertthe M-bit image data to N-bit image data, wherein N is smaller than M;and a lossy compressor to perform lossy compression with respect to theN-bit image data by suppressing fluctuations in a pseudo tone processingfrequency within a certain range.
 2. The image processing apparatusaccording to claim 1, wherein the lossy compressor performs the lossycompression with respect to the N-bit image data for each unit block,and the lossy compressor performs the lossy compression so that, evenafter the lossy compression is performed, the pseudo tone processingfrequency is maintained at a frequency that suppresses fluctuations of amean value corresponding to the M bits, within a certain range in eachunit block.
 3. The image processing apparatus according to claim 2,wherein the lossy compressor performs the lossy compression so that,even after the lossy compression is performed, a difference between themean value of adjacent tones in the M-bit image data is more than 50%.4. The image processing apparatus according to claim 1, furthercomprising: a compression level setting unit to set the compressionlevel of the lossy compressor, and wherein the lossy compressor performsthe lossy compression so that the pseudo tone processing frequency isvariable according to the compression level set.
 5. The image processingapparatus according to claim 1, further comprising: an image sizeacquiring unit to acquire at least one of length and width of the imagedata, and the number of pixels in the image data, as image sizeinformation, and wherein the lossy compressor performs the lossycompression so that the pseudo tone processing frequency is variableaccording to the image size information acquired.
 6. The imageprocessing apparatus according to claim 1, further comprising: a patternarea size acquiring unit to acquire a size of a pattern area in theimage data, and wherein the lossy compressor performs the lossycompression so that the pseudo tone processing frequency is variableaccording to the size of the pattern area acquired.
 7. The imageprocessing apparatus according to claim 1, further comprising: a scalingfactor acquiring unit to acquire a scaling factor for scaling the imagedata; an expansion unit to expand the lossy-compressed image data; and ascaling unit to perform scaling with respect to the image data expanded,based on the scaling factor acquired; and wherein the lossy compressorperforms the lossy compression so that the pseudo tone processingfrequency is variable according to the scaling factor acquired.
 8. Theimage processing apparatus according to claim 1, further comprising: apost-compression processor to perform, after the lossy compression, atleast one process from among outputting the lossy-compressed image data,storing the lossy-compressed image data, and transmitting thelossy-compressed image data; and a pixel density acquiring unit toacquire a pixel density at the time of processing by thepost-compression processor; and wherein the lossy compressor performsthe lossy compression so that the pseudo tone processing frequency isvariable according to the pixel density acquired.
 9. The imageprocessing apparatus according to claim 1, further comprising: acompression level setting unit to set the compression level of the lossycompressor; an image size acquiring unit to acquire at least one oflength and width of the image data, and the number of pixels in theimage data, as image size information; a pattern area size acquiringunit to acquire a size of a pattern area in the image data; a scalingfactor acquiring unit to acquire a scaling factor for scaling the imagedata; and a pixel density acquiring unit to acquire a pixel density;wherein the compression level, the image size, the pattern area size,the scaling factor, and the pixel density are parameters for varying thepseudo tone processing frequency; and the pseudo tone processor performsthe pseudo tone process with respect to the image data, only when atleast one of the compression level, the image size, the pattern areasize, the scaling factor, and the pixel density, is within apredetermined range.
 10. The image processing apparatus according toclaim 4, wherein the pseudo tone processor performs the pseudo toneprocess so that both, the pseudo tone processing frequency and a pseudotone processing amplitude, are variable.
 11. The image processingapparatus according to claim 5, wherein the pseudo tone processorperforms the pseudo tone process so that both, the pseudo toneprocessing frequency and a pseudo tone processing amplitude, arevariable.
 12. The image processing apparatus according to claim 6,wherein the pseudo tone processor performs the pseudo tone process sothat both, the pseudo tone processing frequency and a pseudo toneprocessing amplitude, are variable.
 13. The image processing apparatusaccording to claim 7, wherein the pseudo tone processor performs thepseudo tone process so that both, the pseudo tone processing frequencyand a pseudo tone processing amplitude, are variable.
 14. The imageprocessing apparatus according to claim 8, wherein the pseudo toneprocessor performs the pseudo tone process so that both, the pseudo toneprocessing frequency, and a pseudo tone processing amplitude, arevariable.
 15. An image processing apparatus comprising: a pseudo toneprocessor to perform a pseudo tone process by performing an areacoverage modulation expression process with respect to image data, tothereby reduce a number of bits of the image data; a lossy compressor toperform lossy compression with respect to the image data subjected tothe pseudo tone process; an edge extractor to extract an edge amountfrom the image data before the lossy compression; and an imagesynthesizing unit to synthesize the image data subjected to the lossycompression and the edge amount extracted.
 16. The image processingapparatus according to claim 15, wherein the pseudo tone processorperforms the pseudo tone process so that a total number of the bits ofthe image data after the pseudo tone process and the edge amountextracted, does not exceed a number of the bits of the image data beforethe pseudo tone process.
 17. An image processing method comprising:pseudo tone processing by performing an area coverage modulationexpression process with respect to M-bit image data to convert the M-bitimage data to N-bit image data, wherein N is smaller than M; andperforming lossy compression with respect to the N-bit image data bysuppressing fluctuations in a pseudo tone processing frequency within acertain range.
 18. An image processing method comprising: pseudo toneprocessing by performing an area coverage modulation expression processwith respect to image data to reduce a number of bits of the image data;performing lossy compression with respect to the image data subjected tothe pseudo tone process; extracting an edge amount from the image databefore the performing; and synthesizing the image data subjected to thelossy compression and the edge amount extracted.
 19. A computer-readablerecording medium that records thereon a computer program includinginstructions, which when executed, make a computer execute: pseudo toneprocessing by performing an area coverage modulation expression processwith respect to M-bit image data to convert the M-bit image data toN-bit image data, wherein N is smaller than M; and performing lossycompression with respect to the N-bit image data by suppressingfluctuations in a pseudo tone processing frequency within a certainrange.
 20. A computer-readable recording medium that records thereon acomputer program including instructions, which when executed, cause acomputer to perform a method comprising: pseudo tone processing byperforming an area coverage modulation expression process with respectto image data to reduce a number of bits of the image data; performinglossy compression with respect to the image data subjected to the pseudotone process; extracting an edge amount from the image data before theperforming; and synthesizing the image data subjected to the lossycompression and the edge amount extracted.